The present invention relates to a viewing optical system for use with an endoscope in which light from an object to be examined that is incident upon an objective lens unit is transmitted through an optical-fiber image guide and the resulting focused image is viewed directly with the eye or photographed with a camera.
A prior art viewing optical system for use with an endoscope of the same general type as the present invention is shown in FIG. 1.
An endoscope 1 includes a viewing system 2 composed of an objective lens unit 10 which receives light from an object to be examined under illumination with a lighting optical system (not shown), an optical-fiber image guide 20 that transmits the image of the object focused on the light-receiving end surface 21 via the lens unit 10, and an eyepiece 30 whose focal point coincides with the light-emerging surface 22 of the image guide 20.
The endoscope 1 shown in FIG. 1 is composed of two rigid parts, A.sub.1 and A.sub.2, and one flexible part B. Each of the objective lens unit 10 and the eyepiece 30 is placed in a rigid holder so as to maintain a predetermined distance between the two lens units and a predetermined positional relationship between the objective lens unit 10 (or eyepiece 30) and the end face 21 (or 22) of the image guide 20. The part of the endoscope between the two rigid parts A.sub.1 and A.sub.2 is made flexible so that it can be curved to provide easy access to an object of interest through a channel in the body such as the esophagus. The objective lens unit 10 fitted at the tip 1a of the endscope 1 is composed of a concave lens 11, a convex lens 12 and a doublet lens 13. The function of the doublet lens 13 is to transform the curved image plane formed by the concave and convex lenses 11 and 12 to a flat image plane that conforms to the light-receiving end surface 21 of the image guide 20.
The endoscope 1 described above usually is designed in such a way that the operator can manipulate it on the side of the eyepiece 30 to direct the tip 1a toward a point in directions other than that in which the endoscope is advancing. A greater freedom in changing directions in a limited space can be attained by reducing the length of the rigid part A.sub.1, and this has raised the need to shorten this part.
Attempts have been made to shorten the length of rigid part A.sub.1 by employing fewer lens elements in the objective lens unit 10 and by shortening the overall length of this unit. Those approaches, however, have simply resulted in a deterioration of the focusing performance of the objective lens unit 10, and no clear image of the object can been seen through the eyepiece 30.
For instance, if the doublet lens 13 is eliminated from the objective lens unit 10 in the optical system 2 shown in FIG. 1, the image plane formed by the objective lens unit 10 becomes curved and the image of the object formed on the light-receiving end surface of the image guide 20 is not in focus at all points of the image. That is, if focusing is achieved at the center of the image, marginal portions are defocused.